A twisting apparatus usually comprises a frame on which a plurality of yarn packages and a plurality of spindles are mounted. The yarn is paid off the packages, twisted, and wound up on the spindles. The angular velocity of the spindle and the linear velocity of the yarn or filament being wound up on the spindle determine the amount of twist imparted to the yarn.
As a rule all of the spindles are driven by a common motor whose shaft has a pluraity of pulleys over which are spanned a plurality of flat belts engaging whorls on the spindles. Thus these spindles are driven at a constant speed. A transmission is connected between this shaft and the drive elements that determine the paying-off speed of the yarn from the yarn packages. These drive elements usually comprise driven rollers urged into peripheral contact with the outside of the yarn package and frictionally rotating this package so that the peripheral speed of the package and of the drive elements is equal to the linear velocity of the yarn.
Two types of transmission are used for interconnecting the drive shaft from the fixed-speed motor whose single drive shaft rotates all of the drive elements. These transmissions employ direct gearing so that the drive elements will rotate at a speed that is directly proportional to the motor output speed. In one type the gears are replaceable by gears of different sizes so that different throughput ratios can be obtained and, therefore, the amount of twist can similarly be varied by replacing these gears with gears of different sizes. It is also known to provide a much more complicated and expensive type of transmission which can be shifted by the operation of various clutches and levers so as to change the throughput ratio.
Both of these systems have the considerable disadvantage that only a limited number of throughput ratios can be obtained. The use of a steplessly variable transmission has been ruled out because the inevitable changes in throughput ratios in such transmission caused by thermal expansion, wear, and the like. Any change in throughput ratio changes the amount of twist imparted to the yarn, and, therefore, results in a finished product of nonuniform quality.